1. Field of the Invention
The invention relates to a process for the adsorptive separation of paraffinic hydrocarbons from a feed stream containing a broad admixture of hydrocarbons. More specifically the invention relates to a liquid-phase adsorptive separation process in which the feed stream flows through two different adsorptive separation zones in series and different paraffinic products are recovered from each separation zone.
2. Related Art
The large utility of detergents and other cleaners has led to extensive development in the areas of detergent formulation and production. While detergents can be formulated from a wide variety of different compounds much of the world""s supply is now formulated from chemicals derived from linear alkyl benzenes (LAB). The LAB precursor compounds are produced in petrochemical complexes in which an aromatic hydrocarbon, typically benzene, is alkylated with olefin(s) of the desired structure and carbon number. Typically the olefin(s) is actually a homologous series of different olefins having a range of three to five carbon numbers. The olefin(s) can be derived from several sources. For instance, they can be derived from the oligomerization of C3 or C4 olefins or from the polymerization of ethylene. However, improved detergent characteristics led to the predominating use of straight chain (normal) olefins, and economics has led to the production of these olefins by the dehydrogenation of the corresponding paraffin. These paraffins, typically having 10 to 15 carbon atoms per molecule, are native to kerosene boiling range fraction of crude oils or processed fractions of crude oil. This led to the recovery of these naturally occurring desired paraffins from kerosene becoming the leading commercial source of olefins used in LAB production.
The production of the olefins typically starts with the recovery of paraffins of the appropriate carbon number by adsorptive separation from a hydrotreated kerosene boiling range fraction, which is the subject of this invention. The paraffins are then passed through a catalytic dehydrogenation zone wherein some of the paraffins are converted to olefins. The resultant mixture of paraffins and olefins is then passed into an alkylation zone in which the olefins are reacted with the aromatic substrate. This overall flow is shown in U.S. Pat. No. 2,477,382 issued to A. H. Lewis. A more complete description of this integrated process employing paraffin dehydrogenation and alkylation is shown in an article at page 86 of the November, 1984 edition of Hydrocarbon Processing. 
A description of the use of simulated moving bed (SMB) adsorptive separation to recover paraffins from a kerosene boiling range petroleum fraction is provided in the contents of a presentation made by R. C. Schulz et al. at the 2nd World Conference on Detergents in Montreux, Switzerland on Oct. 5-10, 1986. This shows several incidental steps in the process such as fractionation and hydrotreating. A more detailed overall flow scheme for the production of olefins from the kerosene derived paraffins is presented in U.S. Pat. No. 5,300,715 issued to B. V. Vora.
The success of a particular adsorptive separation is determined by many factors. Predominant in these factors are the composition of the adsorbent (stationary phase) and desorbent (mobile phase) employed in the process. The remaining factors are basically related to process conditions. The subject process preferably employs two adsorbents with one being an adsorbent comprising a molecular sieve referred to in the art as silicalite. The use of silicalite in the adsorptive separation of paraffins is described in U.S. Pat No. 4,956,521 issued to W. K. Volles, which is directed to the production of high octane gasoline blending components. The sequential use of silicalite and zeolite 5A in the separation of monomethylalkanes is described in an article in the Journal of Chromatography, 316 (1984) 333-341. Silicalite has also been described as useful in separating normal paraffins from cyclic hydrocarbons and from branched chain hydrocarbons in U.S. Pat. Nos. 4,367,364 and 4,455,444 issued to S. Kulprathipanja and R. W. Neuzil. The separations described in these patents differs from that performed in the subject process as they correspond to that done in the previously cited article from the World Conference on Detergents, which is performed to recover only normal paraffins.
Several economic advantages are derived from the continuous, as compared to batch-wise, operation of a large scale adsorptive separation processes. Recognition of this has driven the development of simulated moving bed (SMB) adsorptive separation processes. These processes typically employ a rotary valve and a plurality of lines to simulate the countercurrent movement of an adsorbent bed through adsorption and desorption zones. This is depicted, for instance, in U.S. Pat. No. 3,205,166 to D. M. Ludlow, et al.
U.S. Pat. No. 3,510,423 to R. W. Neuzil et al. provides a depiction of the customary manner of handling the raffinate and extract streams removed from an SMB process, with the desorbent being recovered, combined and recycled to the adsorption zone. U.S. Pat. No. 4,006,197 to H. J. Bieser extends this teaching on desorbent recycling to three component desorbent mixtures. U.S. Pat. No. 4,036,745 describes the use of dual desorbents with a single adsorption zone to provide a higher purity paraffin extract.
More recent developments in the area of detergents have led to the discovery that alkylbenzene precursors derived from a mono-methyl paraffin can provide detergents having higher quality. This is described in PCT application WO 99/07656. This reference discloses (pages 7 and 23, FIG. 4) that two adsorptive separation zones may be used in series to produce streams of normal and monomethyl paraffins. Specifically, the feedstream to a second adsorption zone may be the raffinate of an upstream adsorptive separation zone. On page 28 this reference indicates the desorbents may include a lower molecular weight n-paraffin such as heptane or octane. This reference also describes the production of detergents and cleaning compounds from these materials.
The invention is an adsorptive separation process for the recovery of normal acyclic paraffins in which the same desorbent is used in two different adsorptive separation zones recovering different products during the sequential flow of a single feed stream through the two zones. That is, the invention centers on the ability to use a single desorbent of the same composition in both adsorption zones, which reduces the complexity and costs of the overall process. The sequence of the two separation zones may be reversed to yield alternative process flows.
One broad embodiment of the invention may be characterized as an adsorptive separation process to recover both normal paraffins and monomethyl paraffins from a feed stream, which comprises passing a feed stream comprising normal and branched chain and cyclic paraffinic hydrocarbons having between 8 to 23 carbon atoms per molecule into a first adsorptive separation zone, which zone is maintained at adsorptive separation promoting conditions and which contains a first bed of selective adsorbent, selectively retaining a first class of paraffinic hydrocarbons in the first bed of selective adsorbent, and recovering from the first adsorptive separation zone a first raffinate stream comprising a desorbent and branched chain and cyclic paraffinic hydrocarbons which were not selectively retained on the first bed of selective adsorbent and a first extract stream comprising the desorbent and the first class of paraffinic hydrocarbons; passing paraffinic hydrocarbons recovered from either the first extract stream or the first raffinate stream into a second adsorptive separation zone, which second zone is maintained at adsorptive separation promoting conditions and which contains a second bed of selective adsorbent, selectively retaining a second class of paraffinic hydrocarbons on the selective adsorbent of the second bed, and recovering from the second adsorptive separation zone a second raffinate stream comprising the desorbent and paraffinic hydrocarbons which were not selectively retained on the first or second beds of selective adsorbent and a second extract stream comprising the desorbent and the second class of paraffinic hydrocarbons; fractionating the first and second extract streams and the first and second raffinate streams to recover desorbent and producing first and second extract product streams; and passing portions of the thus recovered desorbent into both the first and the second adsorptive separation zones for use in desorbing paraffinic hydrocarbons to produce the first and second extract streams.
A more specific embodiment of the invention is an adsorptive separation process which comprises passing a feed stream comprising alicyclic hydrocarbons, and normal and branched paraffinic hydrocarbons having between 8 to 15 carbon atoms per molecule into a first adsorptive separation zone, which zone is maintained at adsorptive separation promoting conditions and which contains a first bed of selective adsorbent, selectively retaining a first paraffinic hydrocarbon on at least a portion of the first bed selective adsorbent, and recovering from the first adsorptive separation zone a first raffinate stream comprising a desorbent and paraffinic hydrocarbons which were not selectively retained on the first bed of selective adsorbent; passing paraffinic hydrocarbons present in the first raffinate stream into a second adsorptive separation zone, which second zone is maintained at adsorptive separation promoting conditions and which contains a second bed of selective adsorbent, selectively retaining a second paraffinic hydrocarbon on at least a portion of the second bed selective adsorbent, and recovering from the second adsorptive separation zone a second raffinate stream comprising the desorbent and paraffinic hydrocarbons which were not selectively retained on the first or second beds of selective adsorbent; passing a first desorbent stream, comprising the desorbent, into the first adsorptive separation zone and desorbing paraffinic hydrocarbons to produce a first extract stream comprising the first paraffinic hydrocarbon and the desorbent; passing a second desorbent stream, comprising the desorbent, into the second adsorptive separation zone and desorbing paraffinic hydrocarbons to produce a second extract stream comprising the second paraffinic hydrocarbon and the desorbent; fractionating the first and second extract streams to recover the desorbent for reuse in the process.